1. Field of the Invention
The present invention relates to a radio access network system which includes a synchronous server and at least one node such as a base station and a control apparatus operated by a plurality of internal clocks, a radio communication method, a synchronous server and a node.
2. Description of the Related Art
Conventionally, an IP terminal which functions as a synchronous master connected to an IP network periodically transmits synchronous timing packets to an IP terminal which functions as a synchronous slave connected to an IP network. The synchronous slave determines a synchronous state based on a difference between an assumed time of receiving the synchronous timing packets and an actual time of receiving the synchronous timing packets, so as to synchronize a clock of the synchronous slave with a clock of the synchronous master.
Specifically, the conventional IP terminal which functions as the synchronous master transmits the synchronous timing packets to the IP terminal which functions as the synchronous slave at fixed transmission intervals. Clock synchronization is carried out based on a clock counter value during transmission/reception of the synchronous timing packets.
However, the aforementioned synchronization system does not include an IP-based radio access network including a base station and a control apparatus as its application environment, so that the base station and the control apparatus cannot be synchronized with each other in the radio access network, and the out-of-sync state causes an excess or a shortage of transmission/reception data. As a result, a means for solving this problem becomes necessary, and a processing load is increased in order to solve this problem.
According to the synchronization system, the IP terminal is being used as the synchronous master, making it impossible to continuously carry out concentrated control, and possibly causing a stable synchronization processing operation to become difficult.
Furthermore, when a network fluctuation transmission delay occurs between the synchronous master and the synchronous slave, there has been a problem in that an out-of-sync state of the clock occurs between the synchronous master and the synchronous slave due to the influence of the network fluctuation transmission delay in the aforementioned synchronization system.
That is, the synchronous master transmits a synchronous message (a synchronous timing packet) to each of the synchronous slaves in the conventional synchronization system. However, the arrival of the synchronous message at the synchronous slave is delayed depending on a communication state or a communication path, and the delay has been determined as an out-of-sync state of clock irrespective of the fact that the delay is caused by a communication environment. Consequently, it has been difficult to set proper synchronization.